This invention relates to an antilock control system and method for controlling vehicle wheel brakes.
When the brakes of a vehicle are applied, a braking force is generated between the wheel and the road surface that is dependent upon various parameters which include the road surface condition and the amount of slip between the wheel and the road surface. This braking force increases as slip increases until a critical slip value is surpassed. Beyond the critical value of slip, the braking force decreases and the wheel rapidly approaches lockup. Therefore, to achieve stable braking, an antilock braking control system (ABS) seeks to operate wheel slip at or near the critical slip value.
In one known ABS, the brake pressure is modulated by means of a DC torque motor driving a piston in a cylinder whose volume is modulated to control the hydraulic pressure at the wheel brake. The motor is controlled to position the piston at an initial, fully extended home position at which a check valve is unseated to couple the brake system master cylinder to the wheel brake to allow normal braking. When antilock brake pressure modulation is required, the motor retracts the piston (which allows the check valve to close to isolate the master cylinder from the wheel brake) to reduce brake pressure at the wheel brake and thereafter modulates the piston position to provide pressure control for antilock braking. When antilock braking is no longer required, the motor returns the piston to its extended home position. Typically, in these prior systems, the hydraulic pressure at the wheel brake is established based upon the relationship between motor current, motor torque and the hydraulic brake pressure acting on the head of the piston (wheel brake pressure). Motor current as a representation of wheel brake pressure then becomes the controlled parameter to establish a desired braking condition via the brake pressure/motor current relationship.
During ABS controlled braking, when an incipient wheel lock condition is sensed, an ABS cycle is initiated beginning with a pressure release phase wherein the value of motor current at the time the incipient wheel lock condition was sensed is stored as a representation of the brake pressure producing the maximum braking force coexisting with the critical slip between the wheel and road surface and the motor current is controlled to quickly retract the piston to release brake pressure to allow recovery from the incipient wheel lock condition. When a recovery from the incipient wheel lock condition is sensed, a pressure apply phase is initiated in which the motor current is controlled to extend the piston to reapply brake pressure. During the pressure apply phase, brake pressure is initially established at some value related to the previously stored motor current value and the current is then ramped at a controlled rate to ramp the brake pressure at a controlled rate in direction applying brake pressure until an incipient wheel lock condition is again sensed after which the ABS cycle is repeated. In some applications, a pressure hold phase is provided in a region where the wheel is substantially recovered from the incipient wheel lock condition.
In the pressure release phase of this ABS cycle, it is desirable to rapidly reduce brake pressure when an incipient wheel lock condition is sensed in order to prevent the wheel from locking while at the same time reducing the brake pressure only enough to effect wheel recovery from the incipient wheel lock condition and return the wheel to the stable braking region. According, during the release phase of the ABS cycle, it is desirable to quickly release the brake pressure to a value that will affect recovery from the incipient wheel lock condition, but not to some lower pressure value.